RA/Wayland : vertical frequency applied internally but no screen mode switch

Hi ! Trying to minimize latency as much as possible (for cores which are not optimal, like puae, as it doesn't support runahead), I was suggested to switch to Wayland + Vulkan and using a single buffering mode with no VSync.

For some reason, there is no tearing when using single buffering under Wayland. Which means, 20 ms won on 50 Hz cores !

This issue is the following :

- when a core override is set, indicating a custom vertical frequency, it is applied by RA (internal rendering), however it doesn't trigger a monitor mode switch (which remains at the desktop frequency)

(for instance, by default, my monitor is at 144 Hz. I load the UAE core with a 50 Hz freq. override. In the video settings, the current vertical freq. is indeed 50 Hz but the monitor freq. is still 144 Hz)

however, under X, a custom freq. is both applied to RA and triggers a mode switch.

Configuration is : latest SNAP packages, KDE Neon (Ubuntu 20.04 based), Vulkan, AMD GPU, Plasma/Wayland.

Start by tracing how RetroArch applies core-override vertical frequency values under Wayland and compare that path with the working X display-mode switch. Reproduce the configuration with Vulkan, single buffering, and a 50 Hz override on a 144 Hz desktop; done means the monitor mode changes to the override frequency as it does under X.